There are various types of implant materials to be implanted in the living body; for example, devices such as plates, pins and wires made of metals or ceramics are frequently used in the case of osteosynthesis.
However, being extensively high in elastic modulus in comparison with natural bones, these implant materials have a problem of reducing strength of peripheral bones due to a stress reducing phenomenon after healing and are excessive shielding strength. Particularly, in the case of implant materials made of metals, they have problems in that elution of metal ions may exert bad influences upon the living body, sometimes causing a danger of generating carcinogenicity and that, when they are left in the living body for a prolonged period of time after completion of their role such as osteosynthesis, they inhibit natural growth of bones so that it is suitable to carry out re-operation to take out the implant devices from the living body at an early stage after healing such as of bone fracture.
Accordingly, studies have been carried out on biodegradable and bioabsorbable implant materials, and devices for osteosynthesis which are molded with a polyglycolic, a polylactic acid or a copolymer thereof have been developed. Such materials for osteosynthesis, particularly the materials for osteosynthesis made of a polylactic acid, are biocompatible because of their good affinity for the alive body and have a favorable property in that they are gradually hydrolyzed in the living body by the contact with body fluids and finally absorbed by the living body, so that they are frequently used in recent years. In addition, it is not necessary to remove them by re-operation, which is different from the case of the implant devices made of metals.
However, a mini-plate material, etc. made of titanium for use in oral and maxillofacial surgery and brain surgery has an advantage in that it can be used by freely deforming its shape during operation to exert sufficient fixing ability by closely adjusting it to the shape of bone to be treated. Accordingly, in many cases, the same characteristics, i.e., bend-deforming the devices to conform to the shape of the bone upon use, is also in demand for implant devices such as plates for osteosynthesis molded with polylactic acid. As a matter of course, a material prepared to have a flat type shape may be used as such in some cases. Such a plate can be used in the scene of operation by thermoforming it at a temperature of approximately from 60 to 80° C. to adjust it to the shape of the surface of bone to be treated. Although it is a practical method which uses conventional knowledge on the thermoforming of plastics, it requires complex handling.
In general, a molding of polylactic acid having a flat shape such as plate can be easily deformed by bending at ordinary temperature when the thickness is thin. However, when its bending deformation is carried out at an ordinary temperature which is lower than its glass transition point (Tg), whitening occurs in the bending-deformed part portion due to change of the morphology and its strength is reduced, thus causing a problem in that it cannot be used as a plate for osteosynthesis. Thus, in reality, its bending deformation has to be made by heating and softening it as described in the foregoing.
In the polylactic acid implant materials so far developed, uniaxial drawing is carried out by various methods for the purpose of increasing strength, and the polymer molecules and crystals are oriented along the drawing direction by this treatment. At the same time, the polymer becomes fibers when the draw ratio is increased. By the use of their assembled form, a device for osteosynthesis having markedly increased strength of mechanical direction (MD) can be prepared. However, since an implant device in which the polymer molecules are uniaxially oriented in this manner has considerably large anisotropy. Accordingly, the bent part whitens and is easily broken when it is bending-deformed at ordinary temperature by merely a small number of times but to a direction falling at right angle with the orientation direction. It also causes a problem in that it is easily broken when twisted in the orientation direction around the sequence of fibers. Accordingly, it is also difficult to carry out torsional deformation.
In addition, there are other unsolved problems in that, since implant materials solely made of a polylactic acid have no ability to bond to bones, bones cannot be fixed securely because of a possibility to cause loosening after its application to bones. In addition, since they have no bone conductivity, their replacement by bones after degradation and absorption cannot be easily completed.
The present invention was accomplished by taking the aforementioned problems into consideration. The object of the present invention is to provide a biodegradable and bioabsorbable implant devices which have basically large mechanical strength, can be deformed by bending or twisting within ordinary temperature range and can fix and keep the resulting shape as such, has substantially no anisotropy of strength, can be subjected to repeated deformation of exceeding 20 times (can withstand repeated deformation of more than several hundred times in the case of a wire having a circular section) because of its ability of not easily causing whitening and reduced strength by its deformation in any direction partially due to the change of morphology, and also can give a property to bond to bones within a short period of time as well as a bone conductivity.